Which theory attempts to explain observations in the natural world based on evidence and has undergone rigorous testing?

The correct answer is the theory, as it represents a well-substantiated explanation of some aspect of the natural world that is based on a body of evidence collected through observation and experimentation. A theory is formed after a hypothesis has been tested and validated multiple times, making it robust and reliable as an explanation for a particular phenomenon.

Scientists develop theories after extensive testing and gathering of empirical data, which allows them to explain and predict natural occurrences. This rigorous process distinguishes a theory from less formally structured concepts like a hypothesis, which is simply an initial educated guess that has yet to be tested thoroughly, or an assumption, which is a belief accepted without proof. Laws, while also based on robust evidence, typically describe natural phenomena without explaining the underlying reasons. Therefore, a theory encompasses a broader and deeper understanding of the evidence, making it the most suitable answer to the question posed.